Power transmissions, particularly automatically shifted transmissions have a bottom pan or reservoir in which hydraulic fluid is stored. An input driven pump transports hydraulic fluid from the reservoir to the various control apparatus and lubrication passages in the transmission.
During the manufacture of the transmission, it is possible that debris, such as metal chips, can be retained in passages or hollows of some of the components, and flushed into the reservoir after assembly. The inlet to the pump has a filter designed to prevent the debris from reentering the system. However, during operation, if high fluid demand is made or the filter is partially blocked, a bypass on the filter can be opened thus allowing the debris to be recirculated.
While the occurrence of such recirculation is rare, it is preferred to protect against this recirculation. As a general rule, transmissions have been equipped with a magnet in the bottom pan to collect the magnetic particles which are perceived to exhibit the most detrimental affect. There are, however, some nonmagnetic items such as friction facing fibers and aluminum chips found in the transmission. Some secondary filtration has been suggested to control the nonmagnetic debris.